As the circuit density of integrated circuit (IC) chips and multi-chip modules (MCMS) has increased over the past 10 years, flip-chip bonding techniques have been increasingly used to meet the ever increasing interconnect densities between chip and module substrates. In flip-chip bonding, an array of solder bumps is formed on one of the components, usually the IC chip, and this component is turned over to face the second component in a confronting relationship. The two components are then heated to reflow the solder bumps, thereby making the connections between the two components.
One of the major drawbacks in the flip-chip process is the cost and difficulty in forming a dense array of solder bumps. At present, there are a variety of methods employed to form the solder bumps, none of which can form a high density array of solder bumps at a low cost and at a low capital investment of equipment. The evaporation technique, wherein solder is evaporated through a metal mask in an evacuated chamber, requires a high investment in capital equipment and has high cost associated with cleaning the processing equipment and with replacing the metal mask on a frequent basis. The evaporation technique only achieves moderate densities and moderate solder-bump sizes because of the thermal mis-match between the evaporation mask and the substrate.
To achieve higher densities and smaller bump sizes, the electroplating technique is used. In this technique, the substrate surface is covered with an electroplating seed layer, then masked with a photoresist, which is then pattern exposed and developed to form an electroplating mold over each substrate pad, with the seed layer exposed at the bottom of each mold. The seed layer is then electroplated to fill the molds, and the photoresist and the seed layer are thereafter stripped with separate chemical etchants. The electroplating technique is time consuming (due to its many steps), requires high capital expenditures since several pieces of processing equipment are required, and involves hazardous chemicals. Nonetheless, this technique provides the highest bump density and the smallest bump size.
The stenciling technique is the least expensive and requires the least amount of capital expenditure. In this technique, a stencil having apertures therein is placed over the substrate with the apertures overlying corresponding pads of the substrate. As the stencil is held in place, an amount of solder paste is dispensed onto the stencil, and a screening blade (sometimes called "doctor blade") is moved across the stencil surface in such a manner as to force paste into the stencil apertures. The stencil is then removed, which leaves behind bodies of solder paste on the pads, and the bodies are thereafter reflowed to form the solder bumps. This method requires little capital investment, and is comprised by a few quick and inexpensive steps. However, the method cannot achieve small bump sizes and high bump densities for the following reasons. When the stencil is lifted from the substrate, a portion of the solder paste within the aperture sticks to the aperture's side walls and is lifted away from solder paste body. For large apertures, the portion of removed solder paste is a relatively small fraction of the total amount initially deposited in the aperture. However, the fractional amount increases dramatically when the aperture diameter is decreased, and increases to the point where the method is no longer practical. Compounding this problem is the fact that such solder paste stencils have "hour-glass" shaped cross-sections with a constriction of the diameter in the middle aperture's length. The constriction causes a greater amount of paste to be removed.
Accordingly, under the present prior art techniques, one is forced to use expensive and capital-intensive methods to achieve moderate to high density solder bump densities. In order for the flip-chip bonding technology to be commercially successful, a less expensive and less capital-intensive way of achieving high-density solder bumps and small solder-bump size must be found.